The invention relates to air conditioning systems and particularly to the control of the mixing dampers in such systems for regulating both the condition, such as temperature, and the volume of air to be discharged to the zones under control of the system.
The usage of variable air volume systems has attained significant proportions in the recent past, typically in large high-rise buildings, to handle the large interior cooling loads which are relatively constant. These systems generally include an outdoor air duct and an outdoor damper therein for supplying fresh air to the building. This fresh air is mixed with recirculated air through a recirculated air duct and a recirculated air damper and supplied through air treating apparatus to be discharged to the various rooms of the building. The air treating apparatus may include a filter for eliminating dirt particles from the air, a preheat coil for establishing a predetermined minimum temperature of the filtered air particularly in those climates where the outdoor temperature, at least occasionally, drops below freezing, a cooling coil for cooling and dehumidifying the mixed air and a fan for discharging this treated air to the zones or rooms of the building. Each zone, typically, has a temperature sensor connected to a damper for controlling the amount of air supplied to the zone.
The use of outdoor air, recirculated air and exhaust air dampers had long been a standard way of controlling the mixture of outdoor air and return air to provide a controlled temperature of the mixed air. Such systems are designed to take advantage of the cooling capacity of the outdoor air to thereby reduce the amount of energy needed to control the air at the desired conditions. For example, if it is desired to control the temperature within a building at 75.degree. F. and the outdoor air is below 75.degree. F., a large percentage of outdoor air may be introduced into the building rather than having to cool return air to the desired temperature. Therefore, a substantial portion of the outdoor air will be introduced into the building and a substantial portion of the return air will be exhausted from the building.
In the present variable air volume fan systems, the outdoor air, recirculated air and exhaust air mixing dampers are operated to insure that, as the outdoor air damper is opened, the recirculated air damper is closed and the exhaust air damper is opened. Thus, the outdoor air and exhaust air dampers operate in the same sense whereas the recirculated air damper operates in the opposite sense. In these prior systems, the outdoor air, recirculated air and exhaust air mixing dampers control only the temperature of the mixed air such that, if the system is to be a variable air volume system, a fan inlet vane damper or a fan discharge damper or other apparatus must be additionally provided to control the volume of air supplied to the system. If a return fan is used it requires volume control also. The present invention eliminates the need for a fan inlet vane damper or a fan discharge damper or similar apparatus by controlling the outdoor air and recirculated air dampers, and the exhaust air damper if desired, for in turn controlling the condition, such as temperature, of the mixed air and the volume of the mixed air delivered to the zones.